An uplink radio system in a next-generation mobile communication is investigating an introduction of a SC-FDMA (Single-Carrier Frequency Division Multiple Access) technique having a characteristic of a low PAPR (Peak to Average Power Ratio) for a purpose of enlarging a coverage. This SC-FDMA technique is a technique of performing a single carrier transmission by utilizing each divided frequency band responding to a transmission rate necessary for a plurality of the mobile stations.
In this SC-FDMA technique, a CP (Cyclic Prefix) is added to a head of each data block so as to suppress occurrence of interference between the data blocks as shown in FIG. 6. In general, a last part of each data block is copied. A transmission side adds the CP, and a reception side removes this, thereby enabling the interference between the data blocks due to multi-path interference to be prevented from occurring.
The technique of, in the case of performing a demodulating process employing FFT (Fast Fourier Transform) with SC-FDMA technique, controlling a transmission timing of the mobile station so that a timing at which the base station receives the signal transmitted from each mobile station is accommodated within the CP, and performing an FFT operation for the transmitted signals collectively in the base station according to a reference timing has been proposed (Patent technique 1).
One example of a configuration thereof is shown in FIG. 7. This receiving apparatus of SC-FDMA signals is characterized in that one common FFT unit performs the FFT operation for the signals transmitted from all mobile stations collectively.
A CP removal unit 101 has a SC-FDMA received signal and a FFT timing as an input, and removes a portion of the received signal corresponding to GI (Guard Interval) before receipt of the FFT timing, namely, the received signal of CP (Cyclic Prefix).
An S/P converter 102 converts the received signal from which the CP has been removed from a serial signal to a parallel signal.
An FFT unit 103 performs the FFT operation for the SC-FDMA signals of all mobile stations converted to the parallel signals collectively, thereby to perform a conversion from a time domain signal to a frequency domain signal.
A de-mapping unit 104 selects one part of sub-carriers required for receiving process mobile station by mobile station from the output signal of the FFT unit 103, converts a frequency of the carrier of each mobile station to a zero frequency, and makes an adjustment to a sampling frequency according to a transmission rate.
A receiver 105-k (1≦k≦K: K is an integer equal to or more than two) is configured of a reception filter 106 and a demodulator 107.
The reception filter 106 band-limits each mobile station signal within a frequency domain, thereby to separate the mobile station signal, and to suppress noise.
A demodulator 107 is composed of an inverse diffused signal generator 108, a transmission path estimator 109, a noise suppressor 110, a weight calculation unit 111, an equalization filter 112, an IFFT unit 113, a P/S convertor 114, an IFFT unit 115, a P/S converter 116, a delay profile generator 117, and a timing detector 118.
The inverse diffused signal generator 108 has a pilot signal as an input, and generates a diffused signal being employed for estimating a transmission path in the frequency domain.
The transmission path estimator 109 has the received signal band-limited by the reception filter 106 and the inverse diffused signal generated by the inverse diffused signal generator 108 as an input, and calculates a transmission path estimated value in the frequency domain.
The noise suppressor 110 suppresses noise of the transmission path estimated value estimated by the transmission path estimator 109, and improves SNR (Signal Power to Noise Power Ratio).
The weight calculation unit 111 has the transmission path estimated value, being an output of the noise suppressor 110, as an input, and calculates a weight for an equalization filter with MMSE etc.
The equalization filter 112 has the received signal band-limited by the reception filter 106 and the equalization weight calculated by the weight calculation unit 111 as an input, and equalizes the received signal by multiplying the received signal by the equalization weight for each sub-carrier m.
The IFFT unit 113 performs an inverse-fast-Fourier-transform operation for the equalized signal in the frequency domain, being an output of the equalization filter 112, and converts it to a signal in the time domain.
The P/S converter 114 P/S-converts the equalized signal converted to the signal in the time domain, and outputs it as a demodulated signal.
On the other hand, the IFFT unit 115, the P/S converter 116, the delay profile generator 117, and the timing detector 118 detect the timings by employing the transmission path estimated value in the frequency domain outputted from the transmission path estimator 109.
The IFFT unit 115 converts the transmission path estimated value in the frequency domain to the estimated value in the time domain.
The P/S converter 116 P/S-converts the transmission path estimated value represented in the time domain.
The delay profile generator 117 calculates a power of a transmission path response sequence converted by the P/S converter 116, and generates a delay profile.
The timing detector 118 detects a forefront path with a constant level or more from the delay profile generated by the delay profile generator 117, and detects the timing of the above detected path.
Collectively performing a fast-Fourier-transform operation for the SC-FDMA received signals of all mobile stations necessitates synchronizing the timings at which the base station receives the transmitted signals coming from respective mobile stations to one another. For this purpose, the timing signal of each mobile station detected by the demodulator 107 is fed to each mobile station to control the transmission timing mobile station by mobile station.
FIG. 8 shows a block diagram of a general-purpose transmitting apparatus. A TA (Timing Advance) signal, being a difference between the timing information detected by the foregoing timing detector 118 and a reference timing being employed by the base station, is transmitted to each mobile station from the base station.
The TA signal is inputted into a modulator 201 together with other data signals being transmitted through a downlink, a control signal, a reference signal, and so on is subjected to the processes of encoding, interleave, and modulation by the modulator 201, and is outputted to a mapping unit 202.
The mapping unit 202 maps the signal outputted from the modulator 201 on a frequency axis, and output it to an IFFT unit 203.
The IFFT unit 203 performs a conversion from the frequency domain signal to the time domain signal, and outputs it.
The P/S converter 204 P/S-converts the time domain signal for which the inverse-fast-Fourier-transform operation has been performed.
A CP adder 205 adds the CP to the P/S-converted signal, and outputs it. And, a radio transmitter 206 performs the processes such as a D/A conversion, an up conversion, and a transmission power amplification for the above signal, transmits the signals for which the above processes have been executed to the mobile station from a transmitting antenna 207.
FIG. 9 shows a view illustrating an operation of a transmission timing control. Timing detectors 118 in receivers of mobile stations 1 and 2 detects timings T1 and T2 of leading paths, compares the above timings T1 and T2 with a reference timing (TB=0), and feedbacks “TB-T1” and “TB-T2” each of which is a difference between them as TA (Timing Advance) 1 and TA2 to each mobile station, respectively. Respective mobile stations take a control of delaying the transmission timings of T1 and T2 responding to values of the TA being transmitted hereto, respectively. With this operation, the interference between the mobile stations can be avoided because the reception timing of the signal transmitted from each mobile station coincides with the reference timing (TB=0) and orthogonality between the signals transmitted from respective mobile stations is maintained. Further, occurrence of the multi-path interference can avoided because all multi-paths are accommodated within the CP. FIG. 10 indicates a situation of synchronization of the signals received after controlling the transmission timing.
And, this uplink timing control is taken by employing Random Access Channel (RACH) etc. before transmitting an uplink signal. Each mobile station transmits the RACH to the base station, and the base station returns back initial timing information as a response to the RACH to each mobile station. Further, channel resources for transmitting the uplink signals (the data signal, the control signal, and the reference signal) and for transmitting the downlink signals (the data signal, the control signal, the reference signal, and the TA signal) are allocated depending upon a response to the RACH, and a response to a scheduling request.
In addition, the timing update is performed for the mobile station having performed the above-mentioned initial timing synchronization operation from a viewpoint of necessity for securing a synchronization status because a fluctuation in a timing synchronization status occurs due to a fluctuation in the transmission path situation also during the data transmission.
Patent document 1: Japanese patent application No. 2005-280091 (pp. 3-5)